The present invention relates generally to the field of microstrip antennas, and more particularly to planar tunable microstrip antennas for the HF and VHF frequencies.
Microstrip antennas with a lightweight, low profile, low cost and planar structure have been replacing bulky antennas. The length of a rectangular microstrip antenna is about a half wavelength within the dielectric medium under the radiating patch, which is still relatively large at UHF and VHF frequencies, but these frequencies can impose size limitations resulting in bulky and cumbersome antenna structures. Due to the size limitation at UHF and VHF frequencies, previously available microstrip antennas were mainly limited to applications at higher frequencies. The disadvantage of size limitations in UHF and VHF has created a long-felt need to reduce antenna length. Up until now, it has not been possible to employ planar microstrip antennas without the disadvantages, limitations and shortcomings associated with antenna length and size. The present invention makes it possible to fulfill the need for an electrically small planar microstrip antenna for the HF and VHF frequencies.
The long-awaited electrically small planar microstrip antenna for the HF and VHF frequencies offers an number of advantages over prior art antennas. Prior art rectangular microstrip antennas have a half wavelength length within the dielectric medium under the radiating patch, and this is extremely large at UHF and VHF frequencies. The electrically small planar microstrip antenna of the present invention provides the same high efficiency as conventional microstrip antennas, but it also offers a number of key advantages that permit significant decreases in antenna size, without suffering from the size limitations of prior art antenna structures. The present invention also fulfills the long-felt and unsatisfied need for an electrically small antenna for the lower frequencies.
The present invention fulfills the long-standing need for a significantly reduced antenna length and an electrically small antenna for the lower frequencies with a microstrip antenna structure fabricated with a simple microstrip material. This invention""s electrically small planar microstrip antenna also provides the additional advantage of being configured so that it can be easily tunable. The present invention also advantageously provides an antenna with the same high efficiency as quarter wavelength monopole and conventional microstrip antennas, but with an antenna length shortened to less than about 5% of the length of a monopole antenna or conventional microstrip antenna, resulting in small microstrip antennas at low frequencies such as HF and VHF without suffering from the disadvantages, shortcomings limitations of prior art microstrip antennas.
It is an object of this invention to provide a folded multilayer electrically small compact microstrip antenna.
It is another object of this invention to provide a folded multilayer electrically small compact microstrip antenna that permits a substantial reduction in antenna size.
It is yet another object of this invention to provide a folded multilayer electrically small compact microstrip antenna that permits a substantial reduction in antenna size and operates efficiently at low HF and VHF frequencies.
It is still another object of this invention to a simple, low-cost folded multilayer electrically small compact microstrip antenna that permits a substantial reduction in antenna size and operates efficiently at low HF and VHF frequencies.
To fulfill the long-felt and heretofore unsatisfied needs for an electrically small antenna for the lower frequencies and to advantageously attain and accomplish these and other objects the present invention provides a folded multilayer electrically small compact microstrip antenna comprising a folding radiating strip divided into segments interleaved with a multiple layered microstrip dielectric substrate and a means for a conductive ground plane having a number of conductive branches. A narrow portion of the radiating strip, a coaxial connector, and a first conductive branch are positioned so that a wide portion of the radiating strip provides a large junction at the top layer of the multilayer structure. This shortens the length of microstrip impedance transition and greatly reduces the size of the antenna. This impedance transition, in addition to the multilayer structure, provides an extremely shortened antenna length that operates at VHF and HF frequencies. The different embodiments of this invention""s folded multilayer electrically small compact microstrip antenna include 2, 3 and 5 dielectric layers. This invention also encompasses methods for providing substantial reduction in antenna size at the HF and VHF frequencies with a folded multilayer electrically small compact microstrip antenna comprising the steps of interleaving a folding radiating strip, a dielectric substrate interleaved and a conductive ground plane means having several conductive branches.